1. Field of the Invention
The present invention relates to peroxide curable fluoroelastomer compositions. In particular the present invention relates to such compositions that include a siloxane or silazane that contains one or more SiH functionalities. The invention further relates to a method of curing such compositions and to shaped articles obtained from curing such compositions.
2. Background of the Invention.
Fluoroelastomers (elastomeric perfluoropolymers) and in particular perfluoroelastomers are polymeric materials which exhibit outstanding high temperature tolerance and chemical resistance. Consequently, such compositions are particularly adapted for use as seals and gaskets in systems in which elevated temperatures and/or corrosive chemicals are encountered. They are useful in industries such as, chemical processing, semiconductor, aerospace, petroleum, automotive etc.
The outstanding properties of fluoropolymers are largely attributable to the stability and inertness of the copolymerized fluorinated monomer units which make up the major portion of the polymer backbone, e.g., tetrafluoroethylene (TFE), vinylidene fluoride (VDF), hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE) and perfluoro(alkyl vinyl) ethers (PAVE). In order to completely develop elastomeric properties, fluoropolymers are typically crosslinked, i.e. vulcanized. The fluoroelastomers can be ionically cured by addition of suitable curing agents (for instance polyhydroxylic compounds, such as Bisphenol AF or Bisphenol A), and accelerators (for instance ammonium, phosphonium, or aminophosphonium salts), and in the presence of bivalent metals oxides and/or hydroxides (for instance MgO, Ca(OH)2). Peroxide-curable fluoroelastomers have also been developed, which contain iodine and/or bromine atoms along the polymeric chain and/or in terminal position. Such iodine and/or bromine atoms can be introduced by suitable iodinated and/or brominated comonomers, or by using iodinated and/or brominated chain transfer agents during the polymer preparation (see for instance U.S. Pat. Nos. 4,243,770, 4,501,869 and 4,745,165). Curing such polymers in the presence of free radicals, coming for instance from a peroxide, the iodine and/or bromine atoms act as cure-sites owing to the homolytic scission of the carbon-halogen bonds.
Although cured fluoroelastomers with good physical properties can be obtained from these so-called peroxide curable fluoroelastomers, there is a desire to further improve these compositions. U.S. Pat. No. 5,656,697 discloses peroxide curable fluoroelastomer compositions that show improved cure speed and good cross-link density. This is accomplished by adding to the composition certain metal-organic hydrides including for example silylhydride compounds.
U.S. Pat. No. 5,902,857 teaches to add a bisolefin compound to the compositions of U.S. Pat. No. 5,656,697 to improve certain mechanical properties of cured fluoroelastomers obtainable from such compositions. The bisolefin compound is taught to act as a coagent in the curing reaction.
Despite the many different peroxide-curable fluoroelastomer compositions known in the art, it would be desirable to find further compositions having beneficial properties. In particular, it would be desirable to find compositions that allow for easy, cost-effective processing such as, for example, good mold release and reduced mold fouling, as well as compositions that have good curing properties such as speed of cure and cross-link density. Furthermore, the cured elastomers obtainable from such compositions desirably have excellent physical and mechanical properties including high tensile strength, high elongation at break, and low compression set.
The present invention provides a curable fluoroelastomer composition comprising:
(a) a fluoroelastomer comprising a cure site component having a halogen capable of participation in a peroxide cure reaction;
(b) an organic peroxide; and
(c) a siloxane or silazane comprising one or more SiH groups.
In a specific embodiment of the invention, the siloxane or silazane comprises one or more xe2x80x94QSiH groups wherein Q is oxygen (O) or nitrogen (N).
It has been found in connection with the invention that such curable composition can be readily cured at high speed and to a good cross-link density. Moreover, cured fluoroelastomers can be obtained that have good physical and mechanical properties. Further, the compositions generally have improved processing properties, in particular better mold release, less mold fouling, improved skin-cure properties and better flow characteristics.
The invention also provides a method of making shaped articles by curing the curable fluoroelastomer composition and to the shaped fluoroelastomeric articles obtainable from such method.
The siloxanes for use in the curable fluoroelastomer composition are compounds that include one or more Sixe2x80x94O bond and at least one SiH group, preferably an xe2x80x94Oxe2x80x94SiH group. The siloxanes for use in this invention include low molecular weight compounds as well as polysiloxanes which comprise repeating siloxy groups. The polysiloxanes can be linear, branched or cyclic. Examples of low molecular weight siloxanes include for example alkoxy silanes corresponding to the formula:
(Ra)s(RbO)tSiHw
wherein each Ra independently represents an alkyl group such as for example methyl or ethyl or another lower alkyl (C1-C7 alkyl group) or an alkyl group substituted with a substituent such as for example an aryl group, an ester, an alkoxy etc., or aryl group optionally substituted such as for example with an alkyl group, an ester, an alkoxy etc., each Rb independently represents an alkyl group, preferably a lower alkyl group and which may optionally be substituted, t and w represent an integer of at least 1 and the sum of s+t+w being 4. Examples of siloxanes according to the above formula include HSi(OCH2CH3)3 and (CH3)2(CH3CH2O)SiH.
In accordance with another embodiment in connection with the present invention, the siloxane is a polysiloxane (oligomer or polymer), comprising a polysiloxy backbone. Such polymer or oligomer may be terminated by an SiH group and/or may contain SiH groups distributed along the backbone. The SiH groups may form part of the backbone or they can be present in a side group attached to the backbone.
For example, polysiloxanes for use in the curable fluoroelastomer compositions of the invention include those that correspond to the formula: 
wherein R1, R2, R3, R6, R7, R8 and R9 each independently represents hydrogen, an alkoxy group, an alkyl optionally substituted such as for example with an aryl group, an ester, an alkoxy etc., or aryl group optionally substituted such as for example with an alkyl group, an ester, an alkoxy etc., R4 and R5 each independently represents an alkoxy group, an alkyl or aryl group each of which may optionally be substituted, x represents a value of 0 to 150, y represents a value of 0 to 150 and with the proviso that when x=0, at least one of R1, R2, R6, R7, R8 and R9 represents a hydrogen atom.
Specific examples of siloxanes for use in the curable fluoroelastomer composition include 1,1,3,3 tetraisopropyl disiloxane, diphenyl-1,1,3,3-tetrakis(dimethylsiloxy)disiloxane available from United Chem, silylhydride terminated poly(dimethylsiloxane), poly(methyl hydro siloxane) and copolymers of dimethylsiloxane and methylhydrosiloxane.
Further siloxanes for use in this invention may be cyclic such as those corresponding to the formula: 
wherein Rc represents hydrogen, an alkyl group or an aryl group, Rd and Re each independently represents an alkyl or aryl group, i is at least 1 and the sum of i+j is at least 3. Specific examples of cyclic siloxanes according to the above formula are 1,3,5-trimethyl cyclosiloxane and 1-phenyl-3,3,5,5-tetramethyl cyclosiloxane.
Polysiloxanes and siloxanes having OSiH groups are known in the art and can be produced according to well-known procedures such as disclosed in for example: Encyclopedia of Polymer Science and Engineering, Second Edition, V15, Silicones, pgs. 204-308, John Wiley and Sons, 1989. Siloxanes having OSiH groups are also generally commercially available. It will be apparent to one skilled art that the appropriate siloxane or polysiloxane for use in a curable fluoroelastomer composition will dependent on the intended application of the fluoroelastomer and the desired processing conditions thereof. Preferably, the siloxane or polysiloxane will have a molecular weight between 150 g/mol and 10 000 g/mol.
Alternatively to a siloxane or in addition thereto, the curable fluoroelastomer composition may comprise a silazane or a polysilazane. Preferably, the silazane has at least one NSiH group. Suitable silazanes include for example disilazanes corresponding to the formula:
HuSi(Rf)3xe2x88x92uxe2x80x94NRgxe2x80x94SiHu(Rh)3xe2x88x92u
wherein u is 1 or 2, Rf and Rh each independently represents an alkyl group or an aryl group and Rg represents hydrogen, an alkyl group or an aryl group. A specific example of a silazane is HSi(CH3)2xe2x80x94NHxe2x80x94Si(CH3)2H.
The amount of siloxane and/or silazane in the curable fluorelastomer composition is generally between 0.1 and 10 parts by weight for 100 parts by weight of the fluoroelastomer, preferably between 0.5 and 5 parts by weight for 100 parts by weight of the fluoroelastomer, most preferably between 1 and 4 parts by weight for 100 parts by weight of the fluoroelastomer.
The siloxane and/or silazane compound may be included into the curable fluoroelastomer composition as a precursor which during preparation or further processing of the curable fluoroelastomer composition converts into a siloxane or silazene having one or more QSiH functionalities. For example, a chlorosilane may be used as a precursor. Such chlorosilanes are known to hydrolyse in the presence of water into silanols so that a siloxane having one or more OSiH functions may thereby form.
The fluoroelastomers containing a halogen capable of participating in a peroxide cure reaction that can be used in the curable fluoroelastomer composition include well known fluoroelastomers. Typically, the halogen capable of participating in a peroxide cure reaction is bromine or iodine and the amounts thereof comprised in the fluoroelastomer are generally between 0.001 and 5%, preferably between 0.01 and 2.5%, by weight with respect to the total weight of the fluoroelastomer. The halogen capable of participating in a peroxide cure reaction can be present along the chain and/or in terminal position.
In order to introduce halogens, which are capable of participation in the peroxide cure reaction, along the chain, the copolymerization of the basic monomers of the fluoroelastomer is carried out with a suitable fluorinated cure-site monomer (see for instance U.S. Pat. Nos. 4,745,165, 4,831,085, and 4,214,060). Such comonomer can be selected for instance from:
(a) bromo- or iodo- (per)fluoroalkyl-perfluorovinylethers having the formula:
Zxe2x80x94Rfxe2x80x94Oxe2x80x94CFxe2x95x90CF2
wherein Z is Br or I, Rf is a (per)fluoroalkylene C1-C12, optionally containing chlorine and/or ether oxygen atoms; for example: BrCF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2, BrCF2CF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2, BrCF2CF2CF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2, CF3CFBrCF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2, and the like;
(b) bromo- or iodo (per)fluoroolefins such as those having the formula:
Zxe2x80x2xe2x80x94Rxe2x80x2fxe2x80x94CFxe2x95x90CF2
wherein Zxe2x80x2 is Br or I, Rxe2x80x2f is a (per)fluoroalkylene C1-C12, optionally containing chlorine atoms; for instance: bromotrifluoroethylene, 4-bromo-perfluorobutene-1, and the like; or bromofluoroolefins such as 1-bromo-2,2-difluoroethylene and 4-bromo-3,3,4,4-tetrafluorobutene-1;
(c) non-fluorinated bromo- olefins such as vinyl bromide and 4-bromo-1-butene.
The units derived from the cure site comonomer in the final polymer are present in amounts generally comprised between 0.01 and 5% by moles, preferably between 0.1 and 1% by moles.
In replacement of or in addition to the cure site comonomer, the fluoroelastomer can contain a cure site component in terminal position, deriving from a suitable chain transfer agent introduced in the reaction medium during the polymer preparation, as described in U.S. Pat. No. 4,501,869 or derived from a suitable initiator. Examples of useful initiators include X(CF2)nSO2Na with n=1 to 10 (where X is Br or I).
Examples of chain transfer agents include those having the formula RfBrx, wherein Rf is a x-valent (per)fluoroalkyl radical C1-C12, optionally containing chlorine atoms, while x is 1 or 2. Examples include CF2Br2, Br(CF2)2Br, Br(CF2)4Br, CF2ClBr, CF3CFBrCF2Br, and the like. Further examples of suitable chain transfer agents are disclosed in U.S. Pat. No. 4,000,356. The amount of cure site component in terminal position is generally comprised between 0.001 and 3%, preferably between 0.01 and 1%, by weight with respect to the fluoroelastomer weight.
The fluoroelastomer contained in the curable fluoroelastomer compositions according to the invention is generally a polymer that is substantially amorphous, i.e. displaying no or hardly any melting point. Typical molecular weight ranges are from 104 to 5xc3x97105 g/mol. The optimal molecular weight will dependent on the desired application and can be easily determined by one skilled in the art through routine experimentation. The fluoropolymer comprises a major portion of repeating units that are derived from an ethylenically unsaturated and fluorinated monomer, such as an ethylenically unsaturated monomer having a fluorinated double bond (i.e. one or more of the carbon atoms of the double bond carry one or more fluorine atoms). Fluoroelastomers in connection with the present invention include perfluoroelastomers as well as elastomers that are not fully fluorinated. The fluoroelastomers of the curable fluoroelastomer composition can include any of the fluoroelastomers known to those skilled in the art and include in particular those elastomers that are based on VDF as well as those that are based on TFE. Preferred fluoroelastomers are polymers that have repeating units derived from one or more of the following fluoromonomers: vinylidene fluoride (VDF), hexafluoropropene (HFP), chlorotrifluoroethylene, 2-chloropentafluoropropene, fluorinated vinyl ethers such as perfluoroalkyl vinyl ethers (PAVE), e.g. perfluoro(methyl vinyl ether) (PMVE), fluorinated allyl ethers, tetrafluoroethylene (TFE), 1-hydropentafluoropropene, dichlorodifluoroethylene, trifluoroethylene, and mixtures thereof. The aforementioned fluoromonomers may further be copolymerized with other compounds such as with cure-site monomers (CSM) and/or with non-fluorinated alpha olefin co-monomers such as ethylene (E) and propylene (P). Preferred elastomers are copolymers of vinylidene fluoride and at least one terminally ethylenically-unsaturated fluoromonomer containing at least one fluorine atom substituent on each double-bonded carbon atom, each carbon atom of said fluoromonomer being substituted only with fluorine and optionally with chlorine, hydrogen, a lower fluoroalkyl radical, or a lower fluoroalkoxy radical. Specific examples of copolymers include for example copolymers having a combination of monomers as follows: VDF-HFP, VDF-TFE-HFP, VDF-TFE-PAVE, TFE-PAVE and E-TFE-PAVE. It will be understood by one skilled in the art that these polymers are further modified by cure-site monomers (CSM) and/or halogen containing chain transfer agents or initiators so as to include in the polymers the necessary cure-site component(s) for the peroxide curable fluoroelastomer composition.
Further fluoroelastomers for use in the curable fluoroelastomer composition include those that are based on TFE or VDF and that include one or more perfluorovinyl ethers as a comonomer. Examples of suitable perfluorinated vinyl ethers are those of the formula:
CF2xe2x95x90CFO(RfO)n(Rxe2x80x2fO)mRxe2x80x3f
where Rf and Rxe2x80x2f are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and Rxe2x80x3f is a perfluoroalkyl group of 1-6 carbon atoms. A preferred class of perfluoro vinyl ethers includes perfluoro alkyl vinyl ethers of the formula:
CF2xe2x95x90CFO(CF2CFXO)nRxe2x80x3f
where X is F or CF3, n is 0-5, and Rxe2x80x3f is a perfluoroalkyl group of 1-6 carbon atoms. Most preferred perfluoro(alkyl vinyl) ethers are those wherein n is 0 or 1 and Rxe2x80x3f contains 1-3 carbon atoms. Examples of such perfluorinated ethers include perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, and perfluoro(propyl vinyl) ether. Other useful perfluoro vinyl ethers include compounds of the formula
CF2xe2x95x90CFO[(CF2)mCF2CFZO]nRxe2x80x3f
where Rxe2x80x3f is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1, n=0-5, and Zxe2x95x90F or CF3. Preferred members of this class are those in which Rxe2x80x3f is C3F7, m=0, and n=1. Additional perfluoro vinyl ether monomers useful in the invention include compounds of the formula
CF2xe2x95x90CFO [(CF2CFCF3O)n(CF2CF2CF2O)m(CF2)p]CxF2x+1
where m and n=1-10, p=0-3, and x=1-5. Preferred members of this class include compounds where n=0-1, m=0-1, and x=1.
Still further examples of perfluoro vinyl ethers include perfluoro(alkoxy vinyl) ethers:
CF2xe2x95x90CFOCF2CF(CF3)(CF2O)mCnF2n+1
where n=1-5, m=1-3, and where, preferably, n=1.
Specific examples of useful perfluorovinyl ethers include CF2xe2x95x90CFOCF2OCF2CF2CF3, CF2xe2x95x90CFOCF2CF3, CF2xe2x95x90CFO(CF2)3OCF3, and CF2xe2x95x90CFOCF2CF2OCF3.
The fluoroelastomers of the curable fluoroelastomer composition generally comprises from 20 to 85%, preferably 50 to 80% by moles of repeating units derived from VDF, TFE and/or CTFE, copolymerized with one or more other fluorinated ethylenically unsaturated monomer and/or one or more non fluorinated C2-C8 olefins, such as ethylene and propylene. The units derived from the fluorinated ethylenically unsaturated comonomer when present is generally between 5 and 45 mole %, preferably between 10 and 35 mol %. The amount of non-fluorinated comonomer when present is generally between 0 and 50 mol %, preferably between 1 and 30 mol %.
The preparation of the fluoroelastomers for use in the curable fluoroelastomer composition can be carried out by copolymerization of the monomers in aqueous emulsion according to well known methods in the art, in the presence of radicalic initiators (for instance persulphates, perphosphates, or alkaline or ammonium carbonates or percarbonates), optionally with ferrous or silver salts or of other easily oxidizable metals. Surfactants, such as for instance (per)fluoroalkylic carboxylates or sulphonates (for instance ammonium perfluorooctanoate) or (per)fluoropolyoxyalkylenic, or others known in the art, are generally also present in the reaction medium.
When the polymerization is over, the fluoroelastomer is generally isolated from the emulsion by conventional methods, such as coagulation by addition of electrolytes or by cooling. Alternatively, the polymerization reaction can be carried out in mass or in suspension, in an organic liquid wherein a suitable radicalic initiator is present, according to well known techniques. The polymerization reaction is generally carried out at temperatures comprised between 25xc2x0 C. and 150xc2x0 C. under a pressure of up to 10 MPa.
Suitable peroxide curatives for use in the invention are those which generate free radicals at curing temperatures. A dialkyl peroxide or a bis(dialkyl peroxide) which decomposes at a temperature above 50xc2x0 C. is especially preferred. In many cases it is preferred to use a di-tertiarybutyl peroxide having a tertiary carbon atom attached to peroxy oxygen. Among the most useful peroxides of this type are 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexyne-3 and 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexane. Other peroxides can be selected from such compounds as dicumyl peroxide, dibenzoyl peroxide, tertiarybutyl perbenzoate, xcex1,xcex1xe2x80x2-bis(t-butylperoxy-diisopropylbenzene), and di[1,3-dimethyl-3-(t-butylperoxy)-butyl]carbonate. Generally, about 1-3 parts of peroxide per 100 parts of perfluoroelastomer is used.
Another material which is usually blended with the composition as a part of the curative system is a coagent composed of a polyunsaturated compound which is capable of cooperating with the peroxide to provide a useful cure. These coagents can be added in an amount equal to 0.1 and 10 parts per hundred parts fluoroelastomer, preferably between 2-5 parts per hundred parts fluoroelastomer. Because of the use of the siloxane or silazane compound in the fluoroelastomer composition, even low levels of the coagents result in excellent mechanical and physical properties and as a result improved processing of the fluoroelastomer composition can be achieved. Examples of useful coagents include triallyl cyanurate; triallyl isocyanurate; triallyl trimellitate; tri(methylallyl) isocyanurate; tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallyl acrylamide; hexaallyl phosphoramide; N,N,Nxe2x80x2,Nxe2x80x2-tetraalkyl tetraphthalamide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl malonamide; trivinyl isocyanurate; 2,4,6-trivinyl methyltrisiloxane; N,Nxe2x80x2-m-phenylenebismaleimide; diallyl-phthalate and tri(5-norbornene-2-methylene)cyanurate. Particularly useful is triallyl isocyanurate. Other useful coagents include the bis-olefins disclosed in EPA 0 661 304 A1, EPA 0 784 064 A1 and EPA 0 769 521 A1.
Additives, such as carbon black, stabilizers, plasticizers, lubricants, fillers, and processing aids typically utilized in fluoroelastomer compounding can be incorporated into the compositions of the present invention, provided they have adequate stability for the intended service conditions. In particular, low temperature performance can be enhanced by incorporation of perfluoropolyethers (cf. U.S. Pat No. 5,268,405).
Carbon black fillers are used in elastomers as a means to balance modulus, tensile strength, elongation, hardness, abrasion resistance, conductivity, and processability of the compositions. Suitable examples include MT blacks (medium thermal black) designated N-991, N-990, N-908, and N-907, and large particle size furnace blacks. When used, 1-70 phr of large size particle black is generally sufficient.
Oxides and/or hydroxides of multivalent metals, such as Ca, Mg, Pb, Zn, Al, optionally in combination with salts of weak acids are generally added to the curable fluoroelastomer composition. Such oxides and/or hydroxides may be present in the composition in amounts of 1 to 15% by weight, preferably 2 to 10% by weight with respect to the weight of fluoroelastomer.
In addition, fluoropolymer fillers may also be present in the composition. Generally, from 1 to 50 parts per hundred parts fluoroelastomer of a fluoropolymer filler is used. The fluoropolymer filler can be finely divided, easily dispersed plastic fluoropolymer that is solid at the highest temperature utilized in fabrication and curing of the curable fluoroelastomer composition. By solid, it is meant that the fluoroplastic, if partially crystalline, will have a crystalline melting temperature above the processing temperature(s) of the fluoroelastomer(s). Examples of fluoropolymer filler are disclosed in for example EP 708797.
The curable fluoroelastomer compositions may be prepared by mixing a fluoroelastomer, a peroxide curative, a polysiloxane and other additives in conventional rubber processing equipment. Such equipment includes rubber mills, internal mixers, such as Banbury mixers, and mixing extruders.
The curable fluoroelastomer compositions are useful in production of articles such as gaskets, tubing, and seals. Such articles are produced by molding a compounded formulation of the curable composition with various additives under pressure, curing the part, and then subjecting it to a post cure cycle.
The following examples will further demonstrate the present invention. In these examples, the properties were tested as follows.
Cure Rheology Tests were run on compounded admixtures using a Monsanto Moving Die Rheometer (MDR) Model 2000 in accordance with ASTM D 5289-95 at 177xc2x0 C., no preheat, 12 minute elapsed time (unless otherwise specified) and a 0.5xc2x0 arc. Values were obtained for Minimum torque (ML), Maximum torque (MH), i.e., highest torque attained during specified period of time when no plateau or maximum was obtained, were measured. Also reported were: ts2 (time for torque to increase 2 units above ML), txe2x80x250 (time for torque to reach ML+0.5[MHxe2x88x92ML]), and txe2x80x290 (time for torque to reach ML+0.9[MHxe2x88x92MH]). Torque is reported as Newton meters (dNm).
Press-cured samples (150xc3x97150xc3x972.0 mm sheets, unless otherwise noted) were prepared for physical property determination by pressing at about 6.9 MegaPascals (MPA) for the indicated amount of time and temperature.
Post-cured samples were prepared by placing a press-cured sample in a circulating air oven. The oven was maintained at the indicated temperatures and the samples treated for the indicated amount of time.
Force per unit area is reported as Mega Pascals (MPa).
Physical properties were obtained according to ASTM D-412 and Hardness was obtained according to ASTM D 2240.
Compression sets were determined by ASTM D 395-89 Method B with 0.139 inch (3.5 mm) O-rings compressed for 70 hours at 200xc2x0 C. Results are reported as %.